The present invention relates to a process for fabricating an SOI (Silicon-on-Insulator) substrate, using a SIMOX (Separation by Implantation of Oxygen) process, in which a silicon oxide film is formed within a silicon substrate by ion-implanting oxygen into a single-crystal silicon substrate.
As one of the processes for fabricating very-large-scale integrated circuits, the SOI technique has been attracting attention in terms of high integration and low power consumption. Among known processes for fabricating SOI substrates in which a silicon layer is formed on an insulating substrate, are a laser re-crystallization process and a substrate bonding process in which single-crystal silicon substrates are bonded together via an insulating film. Further, the so-called SIMOX process in which oxygen is ion-implanted into a silicon substrate has also been in use.
This SIMOX process is a process in which oxygen ions are implanted to high concentration so that a silicon oxide layer is provided in a specified-depth region of the silicon substrate. In this process, heat treatment is performed for recovery from any damage due to the ion implantation.
As a prior art technique, the process for fabricating SOI substrates is described by referring to Japanese Patent Laid-Open Publication HEI 7-263538.
First, with ion implantation equipment, oxygen ions O.sup.+ are implanted to a specified depth in a single-crystal silicon substrate 21 shown as step (A) of FIG. 2. In this case, in order to avoid any increase in dislocation density in a surface silicon single-crystal layer 22 and any decrease in the strength of breakdown electric field of a buried oxide film 25 shown in step (C) of FIG. 2, the amount of oxygen ion implantation is less than 0.5.times.10.sup.18 cm.sup.-2. In addition, reference numeral 23 denotes a high-concentration oxidized ion implantation layer.
Next, as shown in step (B) of FIG. 2, with CVD (Chemical Vapor Deposition) equipment, an SiO.sub.2 anneal protection film 24 is formed on the surface of the single-crystal silicon substrate 21 (formation of protective film (second step (B))). However, it is also possible to go to the succeeding third step (C) of FIG. 2 without forming the anneal protection film 24.
Next, as shown in step (C) of FIG. 2, the substrate 21 is put into an oven held at 850.degree. C. in an Ar gas atmosphere of a 0.5% oxygen partial pressure, and increased in temperature to 1350.degree. C. (annealing process (third step (C))). Through this annealing process, the substrate is stabilized so that the high-concentration oxygen ion implanted layer 23 changes into the buried oxide film 25. In addition, reference numeral 26 denotes an anneal oxide film.
Next, the single-crystal silicon substrate 21 is subjected to heating treatment for several hours within a temperature range from 1150.degree. C. to less than the melting point temperature (high temperature oxidation process (fourth step)). In this process, the O.sub.2 gas concentration should be held within a range of more than 1% up to 100%.
As a result of performing the above heating process, the following three kinds of improvements are obtained.
First, as shown in steps (D) and (E) of FIG. 2, an increment 27 of the buried oxide film 25 formed in the annealing process is formed as a step for thickening the buried oxide film 25. In addition, reference numeral 28 denotes a surface oxide film increased by high-temperature oxidation.
Also, as shown in steps (F) and (G) of FIG. 2, as a pin-hole reduction step, when particles have deposited on the surface of the single-crystal silicon substrate 21 in the implantation of oxygen ions, pin holes 29 of the buried oxide film 25 are repaired.
Further, as shown in steps (H) and (I) of FIG. 2 for flattening the buried oxide film 25, pits and projections of the top surface of the buried oxide film 25 are flattened by a buried oxide film increment 27. In addition, after the anneal film 26 formed in the above step (C) shown in FIG. 2 is removed, the steps (D) to (I) shown in FIG. 2 may be performed.
Next, as shown in step (J) of FIG. 2, a sacrificing oxidation process is performed with a view to thinning the surface silicon single-crystal layer 22. This sacrificing oxidation may be performed after the removal of the surface oxide film 28. Also, the sacrificing oxidation step may be placed between the annealing process and the high-temperature oxidation process. Furthermore, this sacrificing oxidation process may be carried out after the removal of the anneal oxide film 26.
In the above processes, it is also possible that the annealing process (C) of FIG. 2 is carried out under temperature elevation and, subsequent to the temperature elevation, high-temperature oxidation is performed. Annealing is performed during the process of elevating the oven temperature, where the oven temperature is held constant at a temperature over 1200.degree. C., for example, 1350.degree. C. Then, an increased amount of oxygen is supplied to within the oven after the temperature elevation, so that the internal oxygen partial pressure is adjusted to be a high concentration around 70%. Thus, the high-temperature oxidation process of the fourth step is accomplished.
However, in the process of forming the buried oxide film on the silicon substrate through the annealing process by implanting oxygen ions into a single-crystal silicon substrate, because the silicon oxide film is formed in the silicon substrate by implantation of oxygen ions, there occurs a penetration dislocation to the surface silicon layer. This would incur characteristic deteriorations of transistors and dielectric strength defects of the buried oxide film.